unexpected unnest behaviour

I have a query that blows the arguments in pg_proc out:

SELECT pg_proc.oid, UNNEST(pg_proc.proargnames), UNNEST(pg_proc.proargtypes), UNNEST(pg_proc.proargmodes) FROM pg_proc

And that works great if all arguments are input arguments, but if two are output arguments, then something unexpected happens.

So for a proc declared as:

CREATE FUNCTION "master.dbo".xp_instance_regread(OUT tsql_int, "@root" tsql_sysname, "@key" tsql_sysname, "@name" tsql_sysname, OUT "@value" tsql_sysname)

My query above returns 15 rows instead of the expected 5. When I investigate I find that proargtypes only contains the types of the input arguments and that I should use proallargtypes, and when I do I get the expected results, but the output when unnest is used on arrays of different sizes was something I didn't expect, and something I can't imagine anyone might want...

Mostly out of curiousity, why is this so?

Thanks

James